This invention relates to systems for soldering electrical and electronic components onto substrate boards, and more particularly to an improved apparatus and method for mass reflow soldering electrical and electronic components to the top and/or bottom side surfaces of substrate circuit boards or the like in a single pass. The invention has particular application to soldering of surface mounted devices (SMDs) such as chips or the like to printed circuit boards and will be described in connection with such application.
In response to recent demands for compact electronic appliances and instruments, there has been a tendency to use small, thin chip type electronic parts, i.e. surface mounted components or devices for mounting on printed circuit boards in place of conventional, discrete type, leaded components. The advent of surface mounted component technology has afforded striking new levels of efficiency to the circuit board designer and, at the same time, has posed new challenges for the manufacturing engineer.
Various techniques have been proposed by the art for mass soldering surface mounted components (and/or carriers therefor) to a circuit board. One technique involves fixturing the components to a circuit board, e.g., using an epoxy or the like, inverting the board, and then engaging the inverted board and fixtured components with molten solder by passing the inverted board and fixtured components in contact with a body of molten solder as, for example, through the crest of a standing solder wave. Such a process is described in U.S Pat. No. 4,465,219 to Kenshi Kondo.
It has also been proposed to solder surface mounted components to a circuit board using a solder cream or paste, or a solder preform or the like. In such prior art technique, the surface mounted components are loaded in position on a circuit board, together with suitable solder preforms or a solder cream or paste, and the circuit board and loaded components are heated to a temperature sufficient to reflow the solder. Heating the solder to reflow can be accomplished by baking the circuit board and components in a radiant furnace or the like. Alternatively, the circuit board and components may be immersed in a vapor heated to a temperature above the melting point of the solder, in accordance with the so-called vapor phase soldering or condensation soldering process. Such a process is described in U.S. Pat. Nos. 3,866,307 and 4,321,031, which are given as exemplary. Radiant heating reflow soldering and vapor (condensation) soldering have achieved a certain degree of commercial acceptance although each technique has certain drawbacks. Radiant heating reflow soldering techniques have a tendency to overheat components which could cause damage to heat sensitive components, and radiant heating reflow soldering techniques are susceptible to line-of-sight shadowing and/or thermal absorption differences due to different optical and/or mass characteristics of components and thus may not be entirely satisfactory for many applications. And, the fluid used to create the vapor for condensation soldering is quite expensive, and the thermal decomposition products of the fluid produced are dangerous to health. Moreover, both radiant heating and condensation soldering techniques require relatively long warmup and product changeover lead times.
U.S. Pat. No. 4,600,137 to Matthias F. Comerford entitled METHOD AND APPARATUS FOR MASS SOLDERING WITH SUBSEQUENT REFLOW SOLDERING and assigned to the common assignee of this application discloses an improved mass soldering system which overcomes the aforesaid and other disadvantages of the prior art and also permits mass soldering components to both sides of a printed circuit board in a single pass. More particularly, in accordance with the Comerford patented soldering system, a circuit board containing components populating both top and bottom surfaces of the board is first subjected to a first mass soldering operation in which the bottom side surfaces of the board and components thereon are passed in contact with a body of molten solder as by passing through the crest of a standing solder wave. The first mass soldering operation reportedly also imparts substantial heat energy to the board, which heat energy is conducted through to the top side surface of the board. Additional heat energy then is supplied to the top side surface of the board to reflow preapplied solder cream or paste or solder preforms on the board top side surface. The Comerford patented soldering system comprises a fluxing station wherein flux may be applied to the bottom surface of the circuit board; a preheater station wherein the flux is activated and the board prepared for soldering; a first mass soldering station wherein the bottom side surface of the printed circuit board may be brought into contact with a pool of molten solder; and a second mass soldering station wherein preapplied solder cream or paste or solder preforms on the top side surface of the printed circuit board may be heated to reflow. In a preferred embodiment of the Comerford patented soldering system the first soldering station comprises a wave soldering assembly consisting of two wave-forming solder sumps including a first nozzle for forming a bidirectional wave and a second nozzle for forming a second substantially unidirectional wave, and the second mass soldering station comprises a convection heating assembly consisting of one or more heated nozzles for directing heated fluid streams substantially vertically downwardly onto the top side surface of the board. In a particularly preferred embodiment of the invention the heated fluid comprises heated air. Completing the Comerford patented apparatus is means for transporting the circuit boards to be soldered in-line in timed sequence between the first mass soldering station and the second mass soldering (reflow) station.
The Comerford patented mass soldering system has been incorporated into mass soldering systems available from Hollis Automation, Inc. and is considered to constitute a significant advance and contribution to the art of mass soldering. And, while the above described Comerford patented mass soldering system can be used for mass soldering circuit boards containing only surface mounted components, the Comerford patented mass soldering system is overly complex for soldering circuit boards having no leaded components.
It is thus a primary object of the present invention to provide a mass soldering system, i.e., apparatus and process, which overcomes the aforesaid problems of the prior art.
Another object of the present invention is to provide an improved apparatus and process for mass reflow soldering components to a circuit board.
Still other objects will appear obvious and in part will appear hereinafter.
The invention accordingly comprises the processes involving the several steps and relative order of one or more of such steps with respect to each other and the apparatus possessing the features, properties and relations of elements which are exemplified in the following detailed description and the scope of the application of which will be indicated in the claims.
In the following detailed description of the invention, the term "component" refers to so-called surface mounted components or devices such as chip components as well as carriers therefor. The term "component lead" refers to that part of the metal conductor of an electrical or electronic component that is joined to the printed circuit board pattern. The term "land" as used herein refers to that part of metallic pattern on the printed circuit board to which a component or component lead is joined by solder, the terms "top side surface" and "bottom side surface" as used herein with reference to the circuit board are meant simply to refer to the opposed board surfaces of the circuit board; the terms "top" and "bottom" respectively denote spatial orientation of the circuit board only as it is being processed in the soldering apparatus in accordance with the present invention. The term "mass reflow soldering" is intended to refer to the soldering technique in which solder connections are made by reflow of preapplied solder preforms, or solder cream paste on a circuit board. The term "forced convection" as used herein with reference to the convection heating means is meant to differentiate driven convective heating means as employed in the invention from natural convection movement. The terms "relatively low velocity" and "relatively high velocity" as used herein with reference to the convection heating means are employed as relative terms.
The present invention provides a novel system, i.e. method and apparatus for reflow soldering surface mounted components to one or both sides of a circuit board in a single pass whereby to mechanically and electrically join the components to the circuit board.
More particularly, in accordance with the present invention, there is provided a solder reflow system for mass joining with solder electrical and electronic components affixed on a circuit board and populating one or both surfaces of the board by heating the board and components sufficiently to reflow preapplied solder in the form of preforms and/or solder paste or cream by means of a combination of heat sources, including a first, heating zone comprising a combination of infra-red heating means and relatively low velocity forced convection hot gas heating means for preheating the board and components and leveling the temperature of the preheated board and components to just below the reflow temperature of the solder; and a second heating zone comprising relatively high velocity forced convection hot gas heating means for raising the temperature of the board and components sufficiently high to effect reflow of the solder thereon. In a preferred embodiment of the invention, heat produced by the infra-red heating means in the first preheating zone is captured at least in part and is recycled in the convection hot gas stream. Completing the system are means for transporting the circuit boards to be soldered in-line in timed sequence between the first (preheating) zone and the second (reflow) zone.